Making implantable wireless resonators having small footprints is fundamentally
challenging when using conventional designs that are subject to the inherent
tradeo between their size and the achievable range of quality-factors (Q-factors).
For clinical magnetic resonance imaging (MRI) frequencies (e.g., about 127 MHz
for 3 T), conventional resonators either require a diameter of about 20 cm in chip
size or o -the-chip lumped elements for successful operation, both of which practically
prevent their use as implantable devices. At least two orders-of-magnitude
reduction in footprint area is necessary to make on-chip resonators suitable for invivo
applications. However, decreasing the size of such a conventional resonator
chip comes at the expense of substantially decreased Q-factor. Thus, achieving
high Q-factors with reduced footprints simultaneously entails a novel approach in
implantable electronics. In this thesis work, to address this problem, we proposed,
designed and demonstrated a new class of sub-wavelength, thin- lm loaded helical
metamaterial structures for in-vivo applications including eld localization
and signal-to-noise ratio (SNR) improvement in MRI. This implantable wireless
architecture, implemented fully on chip with partially overlaid helicals on both
sides of the chip interconnected by a through-chip-via, enables a wide range of
resonant radio frequencies tunable on chip by design while achieving an extraordinarily
small footprint area (<< 1 cm2) and ultra-thin geometry (< 30 m).
The miniaturization of such microwave circuits to sub-cm range, together with
their high Q-factors exceeding 30 in lossy soft tissues, allows for their use in vivo.
The fabricated devices correspond to 1/1500th of their operating wavelength in
size, rendering them deep sub-wavelength.For the proposed wireless resonant devices, equivalent circuit models were
developed to understand their miniaturization property and the resulting high
Q-factors are well explained by using these models. Additionally, full-wave numerical
solutions of the proposed geometries were systematically carried out to verify the ndings of the developed equivalent circuit models. All of these theoretical
and numerical studies were found in excellent agreement with the experimental
RF characterization of the microfabricated devices. Retrieval analyses of
the proposed architectures showed that these geometries lead to both negative
relative permittivity and permeability simultaneously at their operating frequencies,
which do not naturally exist together in nature, making these structures
true metamaterials. These fabricated wireless devices were further shown to be
promising for the in-vivo application of subdural electrode marking, along with
SNR improvement and eld localization without causing excessive heating in
MRI. MR images support that the proposed circuitry is also suitable for MRI
marking of implants, high-resolution MR imaging and electric eld con nement
for lossy medium. Although our demonstrations were for the purpose of marking
subdural electrodes, RF characterization results suggest that the proposed device
is not limited to MRI applications. Utilizing the same class of structures enabling
strong eld localization, numerous wireless applications seem feasible, especially
where miniaturization of the wireless devices is required and/or improving the
performance of conventional structures is necessary. The ndings of this thesis indicate that the proposed implantable sub-cm wireless resonators will open up
new possibilities for the next-generation implants and wireless sensing systems.